What is Internal Energy?

Transcription

1 Thermal Energy

2 What is Internal Energy? Individual molecules making up a body have energy Even in a solid, the vibration of the molecules give them kinetic energy The electromagnetic force between the molecules give them potential energy Internal energy is a combination of this kinetic and potential energy

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4 Temperature Temperature is based on the average kinetic energy of the molecules T is therefore independent of mass We define 0ºC as the freezing point of water, 100ºC as its boiling point The Kelvin scale defines 0K as absolute zero: no molecular motion! To convert: 0 K = -273 ºC so K= ºC +273

5 Thermal Energy Thermal energy (Heat) is defined as the transfer of internal energy by: conduction convection, or radiation

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9 Heat Q can be found by Q=mcT where c is a substance s specific heat capacity Ex 1: How much heat is necessary to bring 0.100L of water from 40ºC to 100ºC? Q=mcT =0.100kg(4200J/kgºC )60ºC =25 kj

10 What is the power of a hot plate that heats 0.100L of water in 2.5 minutes? Q=mcT P mct t P 0.10kg P 224W

11 What is the latent heat of vaporization if 25 ml of water are missing after 4 minutes of boiling? Q=mL ml Pt P L t m 224(240) L L J kg 1

12 Heating up lab p. 143 (Gore) Choice of heating devices: Bunsen burner Hot plate Heat lamp? Power calculations Add: measure mass before and after, find heat of vaporization Evaluation: state results, sources of error, possible improvements

13 The mole Avogadro s number tells us how many particles in 12 grams of carbon Ex: how many molecules are in one drop of water? a)10 19 b)10 21 c)10 23 d)10 25

14 Avogadro's number Ex: what is the mass of one mole of water? =18g What is the mass of water that has the double the number of molecules as 11g of quicksilver? Hydrargyrum? Hg?

15 Ex: how many water molecules are in one drop of water (0.1mL) if the density of the water is 1.01g/mL (kg/l)? 1.01g 0.1mL ml mol 18g molecules Choose 6 questions 1-12 p mol molecules

16 We can use E conservation to solve problems involving heat Ex 2: How much heat is generated when we strike an anvil with a 25 kg hammer at 13m s -1? E k =1/2mv 2 =0.5(25)13 2 =2100J Try 3-4 p. 69

17 We can use E conservation to solve problems involving heat Ex 2: How much 75 ºC aluminum will it take to heat 100 ml room temperature water to 65 ºC? E=0 m a c a T a m w c w T w

18 m a=? c a= 903J/kg ºC T a= -10 ºC m w= 0.100kg c w= 4180J/kgºC T w =45 ºC m a c m a a T a m m c w a c w w T c a w T T w w m a 0.10kg J kgc J 45C kgc 10C m 2. 1kg a

19 Ex 3: Mystery object what is the specific heat of a substance that decreases the temperature of 200mL 85 ºC water by 5ºC when you drop in a 0.045kg block at 20ºC? What is it? m x= 0.045kg c x=? T x =60 ºC m w= 0.2kg c w= 4200J/kgºC T w =-5ºC m x c x Q Q x w T x m w c w T w

20 c x m m w x c w T T x w 0.2kg 4200 J 5C kgc c x 0.045kg 60C c x 1600 J kgc

21 Ex 3: Mystery object what is the specific heat of a substance that decreases the temperature of 200mL 77.6 ºC water to 71.4ºC when you drop in a 0.045kg block at 21.5ºC? What is it? m x= kg c x=? T x =49.9 ºC m w= 0.2kg c w= 4200J/kgºC T w =-6.4ºC m x c x Q Q x w T x m w c w T w

22 kj Aluminum 0.91 Gold 0.13 Osmium 0.13 Tantalum 0.14 Antimony 0.21 Hafnium 0.14 Palladium 0.24 Thallium 0.13 Barium 0.2 Indium 0.24 Platinum 0.13 Thorium 0.13 Beryllium 1.83 Iridium 0.13 Plutonium 0.13 Tin 0.21 Bismuth 0.13 Iron 0.45 Potassium 0.75 Titanium 0.54 Cadmium 0.23 Lanthanum Rhenium 0.14 Tungsten 0.13 Calcium 0.63 Lead 0.13 Rhodium 0.24 Uranium 0.12 Carbon Steel 0.49 Lithium 3.57 Rubidium 0.36 Vanadium 0.39 Cast Iron 0.46 Lutetium 0.15 Ruthenium 0.24 Yttrium 0.3 Cesium 0.24 Magnesium 1.05 Scandium 0.57 Zinc 0.39 Chromium 0.46 Manganese 0.48 Selenium 0.32 Zirconium 0.27 Cobalt 0.42 Mercury 0.14 Silicon 0.71 Wrought Iron 0.5 Copper 0.39 Molybdenum 0.25 Silver 0.23 Gallium 0.37 Nickel 0.44 Sodium 1.21 Germanium 0.32 Niobium 0.27 Strontium 0.3

23 c x m m w x c w T T x w 0.2kg 4200 J 6.4C kgc c x kg 49.9C c x 1600 J kgc

24 Mystery metal activity: 1 write-up for each group of 2-3 people Measure 200 ml of cold water into your styrofoam calorimeter Measure the initial temperature of the water and metal Drop the hot metal into the water and seal the container After 2 minutes, measure final temperature, then calculate c

25 Ex 4: unknown T! what will the final temperature be if you drop a 0.45kg block of copper at 340ºC into L room temperature water? m c= 0.45kg c c= 385J/kg ºC T c=? m w= 0.75kg c w= 4180J/kgºC T w =? m c c c m c c c Q Q c w T c m w c w T T T m c T T f ci w w f w wi

26 m c c T c f m c T c c ci m c w w T f m c w w T wi m c c T c f m c w w T f m c T c c ci m c w w T wi m c m c T m c T m c T c c w w f c c ci w w wi T f m c c c T c ci m c c m m w w c c w w T wi

27 m c= 0.45kg c c= 385J/kg ºC T c=? m w= 0.75kg T f m c c c T c ci m c c m m w w c c w w T wi c w= 4180J/kgºC T w =? T f 0.45kg385J / kgc 340C 0.75kg4180J / kgc 0.45kg385J / kgc 0.75kg4180J / kgc 20C T 37C f

28 Do questions 7-11 p. 73

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34 Change of State Chemical bonds have a corresponding energy: Heat of fusion: how much energy is required to melt one kilogram of a material Heat of vaporization: how much energy is required to vaporize one kilogram of a material

35 What s the difference between evaporation and boiling?

36 The next state: Plasma!

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38 Ex: how much heat to vaporize 250mL room temperature water given SLH=2260kJkg 1 First, bring water to boiling point Q=mcT Q=0.25(4180)80 Q=83.6kJ Next, vaporization Q=mL Q=0.25(2260)=565kJ Total=649kJ

39 Do questions p. 75

40 Q=mL Phase Change Ex: how much heat to melt a 5.5 kg block of ice? Q 5.5kg J kg 1 Q 1. 8MJ Try p. 75 Also finish q s p & p

41 Pressure p=f/a Ex: how much pressure does a paper clip exert with its 1mm diameter tip with a force of 50 N? p F r Pa

42 Pressure Ex: how much force does air pressure exert on the roof of our classroom? 11m m F P A Pa 10 5 F N Note: 1 Pascal=1N m -2

43 THERMODYNAMICS: Processes which cause energy changes as a result of heat flow to/from a system and/or work done by/on a system. IDEAL GAS: The molecules obey Newton s laws; The intermolecular forces are negligible; The molecules are spherical with negligible volume; The motion of the molecules is random; The collisions are perfectly elastic; The time taken for a collision is negligible.

44 Ideal gas law Most gases behave like an ideal gas, as long as we are not at extremes of temperature or pressure Based on assumptions of spheres of negligible volume, intermolecular forces PV=nRT

45 Gas Properties PhET Open the Gas properties applet on PhET Choose two variables as your independent and dependent Collect data and graph (then share) PV=nRT

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48 ABSOLUTE ZERO OF TEMPERATURE: The lowest temperature possible C or zero kelvin (0K). The temperature at which the volume, pressure and kinetic energy of an ideal gas are zero. KELVIN TEMPERATURE SCALE: Kelvin is the absolute thermodynamic temperature scale.

49 INTERNAL ENERGY: The energy contained in an object due to the random KE and PE of the molecules. THERMAL ENERGY (HEAT): The non-mechanical transfer of energy between a system and its surroundings. Energy is only heat if it is transferred.

50 Ex: Twice as hot? What is the temperature of nitrogen gas with twice the kinetic energy as room temperature? We convert K=20 ºC +273 Then double: 293x2=586K 586K-273=313 ºC

51 EQUATION OF STATE OF AN IDEAL GAS: Equation which is valid for an ideal gas and many real gases at low pressure. R is the universal molar gas constant (8.31 J mol -1 K -1 )

52 Do questions p. 84

53 P-V DIAGRAMS: Also known as indicator diagrams. The diagram shows how the pressure of a gas varies with its volume during a change. The work done (by or on the gas) is represented by the area under the graph.

54 Isobaric change Constant pressure Isobar W=PV

55 Constant volume Isochoric change

56 Adiabatic change No energy enters or leaves the system

57 Isothermic change Constant temperature

58 Adiabatic changes No heat gained or lost

59 Do questions 19 p. 85 PHeT simulation: Gas properties -collect data, how many of these changes can you graph? Save and the spreadsheet with graphs

60 Engine cycles Work done in one segment = area under graph Total work done = area enclosed by graph

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62 Finish questions p. 84 Do 19 p. 85

63 Assumes ideal gas The Carnot cycle

64 This question is about p V diagrams. The graph below shows the variation with volume of the pressure of a fixed mass of gas when it is compressed adiabatically and also when the same sample of gas is compressed isothermally.

65 This question is about p V diagrams. The graph below shows the variation with volume of the pressure of a fixed mass of gas when it is compressed adiabatically and also when the same sample of gas is compressed isothermally. (a) State and explain which line AB or AC represents the isothermal compression. (b) On the graph, shade the area that represents the difference in work done in the adiabatic change and in the isothermal change. (c) Determine the difference in work done, as identified in (b). (d) Use the first law of thermodynamics to explain the change in temperature during the adiabatic compression.

66 3. (a) pv constant for isothermal / adiabatic always steeper; hence AB; 2 (b) area between lines AB and AC shaded; 1 (c) area is 150 (±15) small squares; (allow ecf from (b)) work done = ^ ^5; = 150 J; 3 For any reasonable approximate area outside the range 150 (±15) squares award [2 max] for the calculation of energy from the area. (d) no thermal energy enters or leaves / Q = 0; so work done seen as increase in internal energy; hence temperature rises; 3 Award [0] for a mere quote of the 1st law.

67 FIRST LAW OF THERMODYNAMICS: The heat supplied to a mass of gas is equal to the increase in its internal energy plus the work done by the gas (expansion is positive work). SYSTEM AND SURROUNDINGS: The fixed mass of gas which is under consideration can be called the system, the place to/from which heat flows is the surroundings.

68 SECOND LAW OF THERMODYNAMICS: No continually working heat engine can take heat from a source and convert it completely into work. OR: Thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature. OR: Although local entropy may decrease, the direction of a process is such as to increase the total entropy of the system and surroundings.

69 PRINCIPLE OF THE CONSERVATION OF ENERGY: Energy may be transformed from one form to another, but it cannot be created or destroyed ie the total energy of a system and its surroundings is constant.

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71 Laws of Thermodynamics

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74 Q=U+W Heat Engine

75 Q=U+W Heat Pump

76 How does a waterwheel work? Water naturally flows from high to low We can use this to get work out If we want to pump the water uphill, we need to put in energy

77 Heat Engines Heat tends to flow from areas of high temperature to low temperature We can use this to get energy out Like a waterwheel, the greater the difference, the more energy we can extract.

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79 Heat pump? If we want heat to flow from cold to hot, we must supply work in This is the first law of thermodynamics W Q t Q L H

80 Q W Q H L Heat Engine Q H W out Q L

81 Q W Q H L Heat Pump Q H W in Q L

82 Do p. 89 Add #22-23 p. 91* Read up to p. 94

83 Entropy The universe is like a teenager: it loves creating disorder, or entropy The 2 nd law of thermodynamics: The entropy of the universe is always increasing Even processes that decrease the entropy of one object still cause total entropy to increase This can be seen as a cause for an arrow of time

84 Can you prove a video is not reversed? What song is this

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86 Entropy This works for Phase change Chemical reactions Gravity? Demolition, Collisions Misc.

87 Entropy